1966. This application Dec. 19, 1969, Ser. No. 882,782 Int. Cl. H05k 3/06 US. Cl. 117212 8 Claims ABSTRACT OF THE DISCLOSURE A blank for the production of metallized articles is provided, which comprises an insulating base which is catalytic throughout its interior to the reception of electroless metal, and a surface on said base having super-imposed thereon and adhered thereto a unitary thin film of metal. In addition, methods for metallizing plastics and forming plated through hole printed circuit boards are also provided.

This application is a continuation of application Ser. No. 561,123, filed June 28, 19166. This application-impart discloses subject matter contained in one or more of the following applications:

This invention relates to materials and techniques for metallizing insulating substrates generally and for the manufacture of printed circuits particularly.

It is an object of the present invention to provide new and improved insulating blanks which are catalytic to the reception of electroless metal and which can be metallized directly, thereby obviating the necessity for seeding and/ or sensitizing.

Another object of this invention is to make rugged and durable metallized objects from such catalytic insulating blanks.

A further object of this invention is to make printed circuit boards from such blanks, including one-layer, twolayer and multi-layer boards.

A further object of this inventionis to make from such blanks printed circuit boards, including one-layer, twolayer and multi-layer boards, which are provided with conductive passageways.

Heretofore, in the manufacture of printed circuit boards comprising conductive passageways or holes through insulating panels, it has been customary to seed and sensitize the lateral walls surrounding the passageways or holes by contacting a perforated substratum sequentially with aqueous acidic solutions of stannous tin ions and precious metal ions, e.g., palladium, or with a single acidic aqueous solution comprising a mixture of stannous tin ions and precious metal ions, such as palladium ions. For example, one such treatment involves immersing the perforated insulating base material first in an acidic aqueous solution of stannous chloride followed by washing, after which the substratum is immersed in an acidic aqueous solution of palladium chloride. In an alternate system, the perforated substratum is simply immersed in a one-step 3,672,986 Patented June 27, 1972 seeder-sensitizer acidic aqueous solution comprising a mixture of stannous chloride and palladium chloride.

Such aqueous acidic seeding and sensitizing solutions have important limitations. Thus, the bond formed between the electroless metal deposit and the surface sensitized with such systems tends to be quite weak and generally unacceptable for many products such as printed circuits, where a strong bond and relatively thick electroless metal deposits are desired. Further, hydrophobic plastics cannot be readily wetted with such solutions and therefore the sensitization achieved with such materials is ordinarily less than satisfactory. An additional disadvantage of such seeding and sensitizing systems is that their use leads to poor and generally unacceptable bonds between the elecrtoless metal and the insulating substrate. When such aqueous seeding and sensitizing solutions are utilized to sensitize lateral walls of the holes or passageways in panels provided with metal foil on one or more surfaces of the panel, the problem of poor bonding between the electroless metal and sensitized areas becomes more acute in the event that'an additional layer of metal is to be superimposed on the original metal foil. This is so because such sensitization of the type described occurs on the exposed metal foil and interferes with the bond between the subsequent metal layer and the initial metal layer. In the manufacture of printed circuits, it is frequently necessary to superimpose additional metal on the foil adhered directly to the substratum for a variety of reasons. Thus, the initial foil may not be thick enough for the desired printed circuit component and additional metal may therefore have to be .added to thicken the pattern. Alternatively, it is frequently necessary to superimpose on the metal cladding a layer of a difierent metal in order to impart special characteristics to the circuit. Typically, metals such as nickel, gold, silver and rhodium, including mixtures of such metals, are electroplated or electrolessly deposited on an initial layer of copper foil or cladding during the manufacture of printed circuits from copper clad laminates. When the aqueous seeding and sensitizing solutions of the type described are utilized in the manufacture of such circuits, the bond between the copper and the metal subsequently superimposed on the copper tends to be weak.

As Will be clear from the following description, use of the catalytic blanks and compositions of the present invention eliminates the need for such conventional seeding and/or sensitizing solutions and therefore eliminates the problems concomitant with the use thereof. Also important is the fact that use of these catalytic blanks and compositions leads to the achievement of uniformly high bond strengths between the insulating substratum and the electroless metal deposit, a result not possible with the conventional aqueous seeding and sensitizing systems of the prior art.

Other objects and advantages of the invention will be set forth in part herein and in part will be obvious herefrom or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.

The invention consists in the novel parts, constructions, arrangements, combinations and improvements herein shown and described. The accompanying drawings referred to herein and constituting a part hereof, illustrate certain embodiments of the invention and together with the specification serve to explain the principles of the invention.

FIGS. 1 and 2 are three-dimensional views of certain embodiments of the blanks of this invention;

FIGS. 3 and 4 are cross-sectional views of further embodiments of the catalytic blanks of this invention;

FIG. 5, A-F, is a schematic illustration of the steps utilized in making a one-sided printed circuit board from the blank of FIG. 1;

FIGS. 6 and 7 are cross-sectional views of typical embodiments of two-sided plated through hole printed circuit boards produced in accordance with this invention utilizing the blanks of FIGS. 2 and 4, respectively;

FIG. 8 is a cross-sectional view of a one-sided plated through hole circuit board manufactured from the blank of FIG. 3;

FIGS. 9 and 10 are cross-sectional views representing the manner in which the blanks of this invention may be combined to form multi-layer printed circuit boards;

FIGS. 9A, 9B, 10A and 10B are cross-sectional views of multi-layer plated through hole printed circuit boards produced by combining blanks as illustrated in FIGS. 9 and 10;

FIGS. 11-17 and 28 are cross-sectional views of additional catalytic blanks produced in accordance with this invention;

FIGS. 11A, 12B, 12C, 14B, 15B, 16B and 17B are cross-sectional views of various articles produced from the blanks of the present invention; and

FIGS. 18-27 illustrate procedures which can be used to produce printed circuit boards from the blanks of this invention.

In the drawings, similar reference numerals are used to represent similar parts.

In FIG. 1 is shown a blank which comprise, in its simplest form, an insulating base 10 having distributed therein an agent 12 which is catalytic to the reception of electroless metal from an electroless metal deposition solution. Hereinafter whenever the term catalytic is employed it will refer to a material which has its propetry, i.e., the ability to receive a metal deposit when exposed to an electroless metal deposition solution, or to catalyze the deposition of metal from such a solution. The catalytic agent 12 may be dissolved in or dispersed throughout the base 10. Alternatively, the insulating base material itself may be catalytic to the reception of electroless metal, e.g., the insulating base material may be formed in whole or in part of an insulating organo-metallic compound which is catalytic to the reception of electroless metal. Superimposed on the base 10 and adhered thereto is a thin unitary and integral metal film or laminate 14 which preferably covers and is substantially conterminous with, i.e., has the same boundaries as, the surface of base 10. The thickness of the metal film 14 will depend primarily upon the manner in which it is fabricated and bonded to the base 10, and will also depend upon the ultimate use to which the blank is to be put. Typically, the metal film will have a thickness of between about 0.05 micron and 105 microns. In a preferred embodiment, the metal film 14 is copper. The thickness of the metal film 14 when made of copper will preferably be such that its weight will vary between about 0.03 and 2. ounces per square foot.

When the metal film 14 is superimposed on the base 10 by means of conventional metal cladding techniques, i.e., by preforming a thin foil of metal, e.g., by electrolytic deposition, and laminating it to the base, the foil 14 will have a thickness of at least about 17 microns. On the other hand, if the metal film is produced by vapor deposition or by the electroless chemical metal deposition technique described herein, it can be as thin as 0.05 micron.

In accordance with a preferred embodiment of the present invention, the film 14 is produced by electroless metal deposition, preferably electroless copper deposition, and has a thickness of between about 0.05 and 30 microns, preferably between about 0.1 and 10 microns. Thin films of the type disclosed having a thickness of less than microns and preferably between 2 and 4 microns, have the ability to be quick etched, as described hereinbelow.

In FIG. 2, there is shown an embodiment of the blank which comprises an insulating member containing a catalytic agent 12. Adhered to both surfaces of the base are thin unitary metal films 14.

FIGS. 3 and 4 illustrate modified embodiments of the blank shown in FIGS. 1 and 2. Thus, in FIG. 3 the catalytic base 10 has superimposed thereon an insulating adhesive resin 18 which is itself catalytic to the reception of electroless metal. The adhesive resin 18 has dissolved therein or dispersed therein a catalytic agent. Alternatively, the adhesive resin 18 may be formed in whole or in part of an insulating organo-metallic compound which is itself catalytic to the reception of electroless metal. The thin layer of metal 14 is adhered to the base 10 by the catalytic adhesive 18.

Similarly, in FIG. 4, the catalytic base 10 is coated on both surfaces with an adhesive 18, which is catalytic, and thin metal films 14 are adhered to both surfaces of base 10 by the adhesive 18.

When certain forms of catalytic agent, e.g., solid particles, are used to prepare the catalytic base 10, there is a tendency for the surface layers of the base 10 to be rich in resin and low in catalyst. As a result, depending upon how the base 10 is manufactured, it sometimes happens that the surface of the base is non-catalytic, even though the interior of base 10 is highly catalytic. This situation is remedied by coating one or both surfaces of the base 10 with a catalytic adhesive 18, as shown in FIGS. 3 and 4. Alternatively, such surfaces could be rendered catalytically active by treatment with acids. Especially suitable are oxidizing acids such as sulfuric, nitric and chromic acids, including mixtures of the foregoing. Treatment with such acids not only renders the surface catalytically active, but it also frequently serves to considerably enhance the bond between the surface and electroless metal deposited there- FIG. 5 illustrates the steps to be used in the manufacture of a one-sided plated through hole board from the blank shown in FIG. 1.

FIG. 5A illustrates the starting blank comprising a catalytic base 10 having a thin metal film 14 adhered to the upper surface. The thin metal film may but need not be conterminous with the upper surface.

In FIG. 513, a negative resin mask 20 has been printed onto the metal foil 14 to leave exposed a positive pattern of the desired printed circuit. At C, FIG. 5, a hole 22 has been provided as by punching or drilling through the foil 14 and base 10, at an interconnecting point of the desired circuit. The blank as it appears in FIG. 5C is then immersed in an electroless metal plating bath of the type described herein to deposit metal 26 on the wall 30 of hole 22. Additional metal 26 deposits on the surface of the metal film 14 which is not covered by the mask 20. If desired, an electrode may be attached to the board after the wall 24 has been formed by electroless deposition, and the circuit pattern and hole walls built up by conventional electrolytic deposition of metal. Following build-up of the circuit to desired thickness either by electroless or electrolytic deposition, the blank is treated with a suitable solvent to remove the mask 20. The blank, following removal of the mask 20, is depicted in FIG. 5E. Finally, the panel is subjected to an etching solution, e.g., ferric chloride, ammonium persulfate, and the like, when the metal film 14 is copper, to thereby remove the thin film of copper 34 which was initially covered by the mask 20. Note that if the metal film 14 is thin, e.g., less than 5 microns, there will be no need to mask the circuit pattern 26 or the plating 24 on the hole walls 30 during the etching step, because the film of metal 14 is so extremely thin compared with the circuit pattern 26 that it will be removed before any substantial etching of circuit 26 or plated wall 24 occurs. Of course, if the initial metal film 14 is thick, the circuit 26 and wall 30 will have to be masked prior to the etching operation.

The etching operation may be carried out by either blasting the surface of the panel with a fine spray of. etchant solution or by immersing the panels, which are held in a rack on a conveyor, in an agitated tank of etchant. During etching, the concentration of the etching solution and the time of contact will be controlled to insure complete removal of thin layer of copper foil in the areas 34. After etching, the panel should be water rinsed to remove all etching chemicals to thereby prevent contamination of the surface or edges of the panels. If desired, the circuit pattern may be plated with additional metals, such as silver, nickel, rhodium, gold or similar high wear resistant materials for special applications. When it is necessary to solder lugs or other hardware to the pattern, it is advisable to solder plate the conductive pattern.

The procedure described above and illustrated in FIG. may also be used to prepare a two-sided, plated through hole printed circuit board of the type shown in FIG. 6, starting with a blank of the type shown in FIG. 2. As shown in FIG. 6, the circuit board comprises a catalytic base having circuit patterns 52 and 54 superimposed on the lower and upper surfaces, respectively. Through connections between the circuit patterns is provided by hole 22, the lateral wall of which is coated with metal 24.

The one-sided plated through hole board of FIG. 8 is prepared by applying the technique illustrated in FIG. 5 and described above to the blank of FIG. 3.

Likewise, the two-sided plated through hole board shown in FIG. 7 is prepared by applying the procedure of FIG. 5 to the blank shown in FIG. 4. In FIG. 7, circuits 52 and 54 on the lower and upper surfaces, respectively, of catalytic base 10 are connected via plated through hole 22, the lateral walls of which are coated with electroless metal 24.

Procedures for producing multi-layer circuit boards from the blanks of the present invention are shown in FIGS. 9, 9A and 9B. In FIG. 9 is shown an embodiment of the invention wherein a blank 500 which consists of a catalytic insulating base 100 having a printed circuit pattern 104 on one surface is laminated to a blank 600 which consists solely of a catalytic resin base 106. Following lamination a circuit pattern 108 (FIG. 9A) may be formed directly on the surface of catalytic base 106 by printing a negative pattern of the circuit with a noncatalytic resinous mask and then subjecting the entire board to electroless metal deposition. If desired, holes 110 could be provided at interconnecting points of the circuit prior to subjecting the laminated structure to electroless deposition, to thereby simultaneous build up a pattern on the surface of catalytic base 106 and plate the lateral walls 112 of the holes 110. The resulting circuit board would look like that shown in FIG. 9A. A circuit pattern '109 could also be formed on the lower surface 101 of catalytic base 100 simultaneously with the circuit pattern 108, to form a board having the appearance of that shown in FIG. 9B.

As has been brought out above, it frequently happens that the catalytic bases described herein comprise resin rich surfaces which are either non-catalytic or poorly catalytic to the reception of electroless metal. To remedy this situation, the multi-layer boards shown in FIGS. 10A and 10B are prepared from the components shown in FIG. 10. Starting with a blank 501 of the type shown in FIG. 10, a circuit pattern 104 is formed by a print and etch technique on the catalytic adhesive 18 which is itself bonded to a catalytic base 100. Next, a blank 502 comprising a catalytic base 106 coated on both surfaces with catalytic adhesive layers 18 is superimposed on the circuit pattern 104. A desirable circuit pattern 150 (FIG. 10A) is then formed on the outer layer of the catalytic adhesive 18 using the additive electroless metal deposition technique described hereinabove in connection with FIGS. 9, 9A and 9B. Here again, holes 110 defining cross-over connections could be provided in the laminated structure prior to subjecting the laminate to electroless metal deposition to simultaneously build up a circuit pattern 150 on catalytic ink 18 and to plate the walls 112 of the holes. A typical example of a resulting multi-layer circuit board formed in this way is shown in FIG. 10A. As will be seen from FIG. 10A, printed circuit patterns 104 and 150 are adhered to catalytic base members and 106, respectively, by catalytic adhesive 18. The entire assembly is also held together with catalytic adhesive resin ink 18. Holes plated with metal 112 provide through connections between circuits and 104. It will be appreciated that the use of the catalytic ink layers 18 in the multilayer embodiment of FIGS. 10 and 10A insures against a discontinuity in the lateral wall 112 of the plated through hole 110 adjacent the point at which the separate layers of the circuit are joined to their respective bases. If desired, a circuit pattern 151 could also be formed on the surface 101 of catalytic base 100 simultaneously with the circuit pattern 150 to form a multi-layer board having the appearance of that shown in FIG. 10B. In this embodiment, an additional layer of catalytic ink 18 will preferably be used to coat surface 101 prior to producing the circuit 151 by the additive electroless metal deposition technique described hereinabove.

It should be appreciated that in multi-layer embodiments of the type shown in FIGS. 9A, 98, 10A and 10B, all circuit patterns could be formed by the additive technique described herein. Similar, as already brought out, all circuit patterns of such embodiments could be formed by the print and etch technique.

Additional catalytic blanks for use in making printed circuits of the type described are shown in FIGS. 11-17.

It is sometimes desirable in single-sided, double-sided and multi-layer boards, to have one surface of the finished board completely non-catalytic. Blanks suitable for making such boards are shown in FIGS. 11-15.

Thus, in FIG. 11 there is shown a blank which consists of a catalytic insulating base 10 which has a noncatalytic insulating surface 11 either bonded thereto or integral therewith. The non-catalytic insulating surface 11 will ordinarily be conterminous with the adjacent surface of the base 10. In FIG. 12 is shown a blank which comprises a catalytic insulating base 10 having non-catalytic insulating surfaces 11 either bonded to or integral with both surfaces of the base 10. Here again, non-catalytic insulating surfaces 11 will ordinarily be conterminous with the adjacent surfaces of base 10.

In FIG. 13 is shown a blank comprising a catalytic insulating base 10 comprising a conterminous lower noncatalytic insulating surface 11. Adhered to the upper surface and preferably conterminous therewith is a thin film of metal 14.

In FIG. 14 is shown a blank useful for the manufacture of printed circuit components which comprises a catalytic insulating base 10 having one non-catalytic insulating surface 11 conterminous therewith. The opposite surface of the catalytic base member 10 comprises a catalytic insulating adhesive layer 18 on which is superimposed a thin metal film 14.

In FIG. 15 is shown still another embodiment of the blank of this invention which comprises a catalytic insulating base 10 having one insulating surface 11 which is non-catalytic and a second insulating surface 18 which comprises an insulating catalytic adhesive of the type described herein.

Additional blanks which are suitable for use in the preparation of printed circuits or generally in the metallization of plastic substrates are shown in FIGS. 16' and 17. In FIG. 16, there is shown a blank which comprises an insulating catalytic base 10 having one surface which comprises a catalytic insulating adhesive 18.

In FIG. 17, there is shown another blank which comprises a catalytic insulating base 10, both surfaces of which comprise a catalytic insulating adhesive 18. The blanks of FIGS. 16 and 17 are particularly useful in forming the multilayer boards shown in FIG. 10.

Preferably, in those embodiments of the invention calling for a catalytic adhesive 18, the adhesive will take the form of a flexible adhesive resin of the type described hereinbelow. The flexible adhesive resins which are catalytic to the reception of electroless metal and are also insulating in nature, insure a strong reliable bond between the circuit pattern and the catalytic insulating base.

As will be appreciated from the foregoing, all of the blanks described herein may be used to form metallized insulating substrates directly on insulating base materials without the necessity of seeding the insulating material prior to metallization.

A distinct advantage of these blanks in printed circuit manufacture is that they can be used to produce directly rugged and reliable printed circuit boards having plated through holes. Use of such blanks eliminates the preseeding and/or pre-sensitizing steps of conventional practice together with the concomitant problems associated with such practice.

Catalytic insultaing bases containing non-catalytic surfaces may be made in a variety of ways. Thus, the catalytic insulating base could be made with a minimal amount of catalytic agent to insure that the surface of the base is extremely rich in insulating and extremely poor in catalyst. When formed, such a base, or laminates impregnated with such a base, will have surfaces which are substantially non-catalytic to the deposition of electroless metal.

Alternatively, a catalytic insulating base rich in catalyst could be prepared and one or both surfaces thereon then coated with a non-catalytic insulating film or adhesive. For example, when the catalytic base is made by impregnating paper or fibrous substrata, e.g., Fiberglas, with catalytic resin, a final gel coat of non-catalytic resin could be superimposed on the laminated structure during manufacture to produce the non-catalytic surface. Alternatively, a film of non-catalytic resin could be bonded to the substrata following completion of lamination.

In the manufacture of the catalytic base materials and adhesives described, an agent which is catalytic to the reception of electroless metal is distributed throughout an insulating base or adhesive, as by dissolution, dispersion, Or by reacting a part or all of the material of the base or adhesive with a catalytic agent so as to form a chemical compound or complex, which is itself catalytic to the reception of electroless metal. The resulting base or adhesive will be catalytic to the reception of electroless metal throughout its interior.

Exposed surfaces of the catalytic base materials of this invention are catalytic to the reception of electroless metal, or may be rendered catalytic by subjecting the surface to relatively mild mechanical or chemical abrasion or etching or by coating the surface with catalytic adhesives of the type described.

A film of metal as shown in FIGS. 1-4, accordingly, may be readily superimposed on such a base simply by immersing the base in an electroless metal deposition solution of the type to be described. Alternatively, the catalytic base could actually be clad with a thin metal foil, using typical metal cladding or lamination techniques, e.g., by bonding a thin foil of metal to the base.

Alternative procedures for making multi-layer printed circuits from a metal clad insulating catalytic base by the so-called print and etch technique are shown schematically in FIGS. 18-27. These embodiments are suitable for use with blanks in'which a thick metal foil is clad to a catalytic base. Preferably, however, the techniques of these figures will be practiced with a catalytic base material clad with a thin metal foil, e.g., less than 30 microns, and preferably less than 5 microns, in thickness.

At A in FIG. 18 there is shown a metal clad laminate having an insulating catalytic core or base covered by a thin metal foil 14.

At B the laminate is printed by means of a step and repeat negative 16 with an acid resist material 15.

The appearance of the laminate following printing is shown at C. Following printing, the foil not protected by the acid resist is etched, to form a conductor pattern 14-15 shown in FIG. 18D. Following etching, the resist 15 is removed to leave a first conductive pattern of metal foil 14 adhered to base 10 as shown in FIG. 18E. In FIG. 18F, a layer of catalytic insulating resin 19 is superimposed on the base 10 and circuit pattern 14. As

shown in FIG. 186, a negative mask 17 is next printed on the catalytic ink 19 to leave exposed a positive pattern 9 of a second printed circuit. Next, holes 22 are provided in the panel at interconnecting points, as shown in FIG. 18H. Finally, the panel is immersed in an eletcroless metal deposition solution to deposit electroless metal 24 on the walls surrounding the holes 22 and on the exposed pattern 9 of catalytic ink 19 to form a second circuit pattern 54. The resin mask 17 may be a permanent mask or may be removed following electroless metal deposition. The printed pattern may be formed on the metal clad blanks of this invention in a variety of Ways.

In the so-called photographic technique, the surface is cleaned and degreased, and a light sensitive enamel is uniformly spread over the metal foil and dried.

The photographic system of printing could also be used to produce the mask in the additive process for producing a circuit pattern by eletcroless metal deposition techniques described hereinabove. Whenever required, the light sensitive enamel could be made catalytic to the reception of electroless metal by dissolving or dispersing therein an agent which is catalytic to the reception of electroless metal.

For long production runs, the photographic system of printing tends to be slow and expensive, and as a result, etch resist printing will ordinarily be carried out either by offset printing on an offset printing press or by screen stencil printing on a manual or automatically operative screen printing press. The step and repeat negative is used to produce, in the case of an offset printing press, an offset printing plate. Acid resist ink is transferred by a rubber covered roll from the printing plate to the metal clad base.

In screen printing, the step and repeat negative is used to produce a stencil on the silk or wire mesh of the screen frame. The stencil is made photographically from the negative and reproduces it exactly.

Regardless of the type of printing employed, it will be understood that either a positive or a negative image of the desired conducting patterns may be imposed on the base, with suitable modifications to insure that the final conductive pattern desired is ultimately obtained.

When offset or screen stencil printing is employed, the ink used in printing is acid resistant, so that the portions of the metal foil recovered thereby are not affected by the etching solution when the plate is contacted therewith. Such acid resistant inks are well understood in the art, and commonly comprise resins such as cellulose acetate, cellulose butyrate, casein-formaldehyde, styrene-maleic anhydride, and the like. Such materials are acid resistant but can be readily removed when desired by readily available solvents or otherwise.

One etching solution commonly used with copper clad stock is ferric chloride. The etching opertion is carried out by either blasting the surface of the panel with a fine spray of ferric chloride or immersing the printed sheets, which are held in a rack or on a conveyor, in an agitated tank of ferric chloride. The etching operation is controlled by the concentration of the etching solution and time of contact, and these variables must be carefully controlled empirically for good results. After etching, a Water rinsing process is employed to remove all etching chemicals, thereby preventing contamination of the surface or edges of the panel.

Frequently, a bare copper foil circuit is not adequate. If, for example, the circuit pattern is to be used as a switch, slip ring, or commutator, it may be necessary to plate the circuit pattern with silver, nickel, rhodium, gold and similar highly wear resistant metals. Where it is necessary to solder lugs or other hardware to the pattern, it may be advisable to have the conductor pattern solder plated.

The steps in an alternative process for making two-sided plated through hole printed circuit boards using the metal clad insulating catalytic bases of this invention are described schematically in FIG. 19.

In FIG. 19A is shown a blank comprising a catalytic base clad on both surfaces with metal foil 14. In FIG. 19B, a positive pattern of the desired circuit is made on the surface of the blank by printing a positive pattern of the desired circuit on each surface with an etch resistant ink 15. In FIG. 190, the metal on both surfaces in the area not covered by the mask has been etched to remove the metal foil. In FIG. 19D, the etch resist has been removed and the panel has been coated on both surfaces with an insulating, non-catalytic mask coating 17. Holes or apertures 22 are then made in the panel as shown in FIG. 19E. Any suitable procedure such as punching, drilling, etching, and the like, may be used to make the holes 22. The panel is then subjected to electroless deposition for a suitable period of time to form an adherent deposit of electroless metal 24 on the lateral walls of holes 22 to thereby connect the circuit patterns on both sides of catalytic base 10, the finished circuit appearing as shown in FIG. 19F. If desired, the mask 17 may be removed to form, as the finished circuit, the two-sided plated through board shown in FIG. 19G.

FIG. 24 illustrates the steps to be followed utilizing the FIG. 19 procedure to make a four-layered board from the blanks shown in FIG. 24A and FIG. 24B. In FIG. 24, reference numeral 10 in a catalytic insulating base 14 is a thin film of metal adhered to said base, 201 is an insulating, non-catalytic resinous mask, 110 is a hole, and 112 is a deposit of electroless metal connecting the walls of holes 110.

Another embodiment of the present invention is described schematically in FIG. 20.

In FIG. 20A is shown a blank comprising a catalytic base 10 clad on both surfaces with metal foil 14 and provided with apertures or holes 22 at pre-selected points. In FIG. 20B the metal clad stock containing apertures 22 is exposed to an electroless metal deposition solution to form a thin, uniform deposit of electroless metal on the foil 14 and on the lateral wall 24 surrounding the hole. In FIG. 200, the blank has been printed with an etch resist pattern 36 using the photographic technique described hereinabove. The etch resist 36, it will be noted, extends through the holes 22 and protects the electroless metal deposit in the holes. In FIG. 20D, the blank has been etched to form the circuit pattern with plated through holes. In FIG. 20E, the etch resist pattern 36 has been removed to form the completed circuit. In the embodiment of FIG. 20, after the walls 24 of holes 22 have been formed by electroless deposition, the thickness of the circuit pattern and the plated through hole could be built up by standard electrolytic techniques, if desired. For example, a negative mask could be imposed on the surface of the blank following the electroless deposition of step B, and the blank subjected to electrolytic deposition to build up the circuit pattern.

A further method of forming two-sided plated through hole printed circuit boards is shown schematically in FIG. 21.

In FIG. 21A is shown a blank comprising a catalytic base 10 clad on both sides with metal foil 14. The catalytic base 10 has been prepared or suitably treated to insure that its upper and lower surfaces are not catalytic to the reception of electroless metal. If desired, a blank of the type described in FIG. 12, metal clad on both surfaces could be used as the base 10 in the FIG. 21 embodiment. The blank is printed with a positive pattern of etch resist 36 as shown in FIG. 21B. Following printing, the plate is etched to leave the conductive portions of the pattern intact, the remaining portion of the foil having been etched away as shown in FIG. 21C. The etch rsist 36 is then removed so that the panel looks as shown at D in FIG. 21. Following removal of the etch resist, the panel is im mersed in the electroless plating bath to deposit a uniform deposit of electroless copper 38 on the foil 14 and on the walls surrounding the holes as shown in FIG. 21E.

In FIG. 22 is depicted schematically the sequence of steps in the formation of a four-layered plated through hole board utilizing the procedure of FIG. 18 as described above. Since the reference numerals of FIG. 22 are identical to those of FIG. 18 and since the procedure of FIG. 22 is identical to that of FIG. 18, the procedure used in FIG. 22 is self-explanatory.

In FIG. 23 is shown still another embodiment of making plated through hole printed circuit boards using blanks of the type described. In FIG. 23A is shown a blank comprising a catalytic base 10 provided with holes 22. A negative pattern of the desired printed circuit is printed on the base 10 with an insulating ink. Negative mask 7 is noncatalytic. The blank is then subjected to electroless metal deposition to deposit a thin film of electroless metal 5 on the portion of the upper surface of the base not covered by the mask 7, on the walls surrounding the holes, and on the lower surface 1 of the base 10.

Lower surface 1 of the base 10 is then masked with a resist 71 as shown in FIG. 23C and the blank then connected as an electrode in an electrolytic metal deposition solution to build up the circuit pattern 5 electrolytically as shown at 69 (FIG. 23D). Alternatively, the pattern could be built up by electroless metal deposition. Following build-up of the circuit pattern, including the walls of the holes, the masks 71 and 7 are stripped from the blank and the blank subjected to a mild etch to remove the thin film of electroless metal 5 remaining on the lower surface 1. The finished circuit board following stripping i shown in FIG. 23D.

In FIG. 25 is shown a schematic illustration of the steps which could be used to produce printed circuits following a modified embodiment of the FIG. 23 procedure. In FIG. 25A is shown a blank which consists of a catalytic base 10 clad on both surfaces with a thin, e.g., less than 1 micron, metal film 14. Holes 22 are provided in the blank at pre-selected cross-over points. In FIG. 25B the blank has been coated on its lower surface with a noncatalytic resinous mask 601. A negative image of the desired circuit pattern has also been printed on the top surface of the blank as shown at 601. The next step in the procedure is to expose the blank to an electroless metal solution, thereby depositing electroless metal 24 on the walls surrounding the holes and also on the areas of the upper metal film 14 not covered by the mask 601, thereby imposing a circuit pattern 602 on the top surface of the blank. Next, if desired, the blank could be hooked up as an electrode in an electrolytic metal deposition solution to deposit additional metal 24A on the walls surrounding the holes and also to build up the circuit pattern 602 as shown at 602A. When the circuit pattern and the Walls have been built up to the desired thickness, the blank is subjected to a suitable solvent to remove mask 601. Next, the blank is subjected to a suitable etchant to remove the thin layer of metal 14 on the lower surface of catalytic base 10, and on the upper surface of base 10 in the areas previously covered by mask 601. Following etching the completed circuit will have the appearance indicated in FIG. 25D.

Printed circuit boards depicted in FIG. 11A could be made from the blank of FIG. 11. Thus, a negative mask of the circuit could be superimposed on the upper surface 41 of the catalytic base 10 shown in FIG. 11. Holes defining cross-overs, if desired, could be made in the base 10. The entire blank would then be exposed to an electroless metal deposition solution to deposit electroless metal on the area of surface 41 not covered by the mask and on the lateral walls of the holes, following which the mask Would be removed. The finished circuit board is depicted in FIG. 11A, wherein 51 represents the printed circuit pattern which includes holes 22 with plated walls 24. The board has an insulating, non-catalytic base 11.

The blank of FIG. 12 could be used to make plated through hole boards of the type shown in FIG. 12B. The top and bottom surfaces 11 of the circuit of FIG. 12B are non-catalytic, as brought out hereinabove. The circuit of